There is known a background image forming apparatus which forms a toner image on a surface of a drum-shaped photoconductor through a well-known electrophotographic process.
The structural configuration of such an apparatus is as follows. An endless intermediate transfer belt is brought into contact with the photoconductor to form a primary transfer nip. In the primary transfer nip, the toner image on the photoconductor is primarily transferred onto the intermediate transfer belt. A secondary transfer roller is brought into contact with the intermediate transfer belt to form a secondary transfer nip. In the loop of the intermediate transfer belt, a secondary transfer opposite roller is disposed. The intermediate transfer belt is nipped between the secondary transfer opposite roller and the above-described secondary transfer roller. The secondary transfer opposite roller disposed inside the loop is electrically grounded. By contrast, a secondary transfer bias is applied to the secondary transfer roller disposed outside the loop. Between the secondary transfer opposite roller and the secondary transfer roller, therefore, a secondary transfer electric field is generated which electrostatically moves the toner image from the side of the secondary transfer opposite roller toward the side of the secondary transfer roller. The toner image on the intermediate transfer belt is secondarily transferred onto a recording sheet conveyed into the secondary transfer nip in synchronization with the toner image on the intermediate transfer belt.
In the above-described configuration, with recording media with substantial surface roughness, such as a Japanese paper sheet, an uneven toner image density pattern conforming to the surface roughness tends to be formed in the toner image, owing to a failure to transfer a sufficient amount of toner to recesses in a surface of the sheet.
Accordingly, the background image forming apparatus employs, as the secondary transfer bias, a superimposed bias including an alternating current (AC) voltage component superimposed on a direct current (DC) voltage component, instead of a bias including only a DC voltage. It has been shown experimentally that it is possible to minimize the formation of an uneven density pattern conforming to the surface roughness of the recording sheet by employing the secondary transfer bias including a superimposed bias.
However, the present inventors have found from experiments that, depending on the voltage condition of the secondary transfer bias, the formation of an uneven density pattern is either insufficiently minimized or, if minimized, white spots attributed to uncontrolled electrical discharge appear in the image.
The above issues are described in further detail below with reference to the configurations shown in FIGS. 1 and 2. FIG. 1 is an enlarged configuration diagram of a related-art image forming apparatus 530 illustrating an example of the secondary transfer nip. As shown in FIG. 1, an intermediate transfer belt 531 is pressed against a nip formation roller 536 by a transfer inner surface roller 533 in contact with an inner surface of the intermediate transfer belt 531. With this pressing, a transfer nip is formed in which an outer surface of the intermediate transfer belt 531 and the nip formation roller 536 come into contact with each other. A toner image on the intermediate transfer belt 531 is transferred onto a recording sheet P conveyed into the transfer nip. A transfer bias for generating a transfer electric field for transferring the toner image is applied to one of the two rollers illustrated in the drawing, and the other roller is electrically grounded. It is possible to transfer the toner image onto the recording sheet P, irrespective of which one of the rollers is supplied with the transfer bias. Herein, a description is given of a case of applying the transfer bias to the transfer inner surface roller 533 and using toner of negative polarity. In this case, to move the toner in the transfer nip from the side of the transfer inner surface roller 533 toward the side of the nip formation roller 536, a bias having a time-averaged electric potential of the same negative polarity as the polarity of the toner is applied as the transfer bias including a superimposed bias. In a case in which toner of negative polarity is used and a transfer bias is applied to the nip formation roller 536, it is necessary to employ a transfer bias having a time-averaged potential of positive polarity, that is, a polarity that is the opposite of the polarity of the toner.
FIG. 2 is a waveform chart illustrating an example of a waveform of the transfer bias including a superimposed bias and applied to the transfer inner surface roller 533. In the drawing, an offset voltage Voff in volts (V) represents the time-averaged value of the potential difference between the transfer inner surface roller 533 and the nip formation roller 536. In the illustrated example, the nip formation roller 536 is electrically grounded. Therefore, the value of the offset voltage Voff is substantially equal to the value of the DC component of the transfer bias. As illustrated in the drawing, the superimposed bias has a sinusoidal waveform, and includes a positive peak value and a negative peak value. A reference sign Vt represents one of the two peak values for moving the toner in the transfer nip from the intermediate transfer belt side toward the recording sheet side, i.e., the negative peak value in the present example (hereinafter referred to as the transferring peak value Vt). A reference sign Vr represents the other peak value for returning the toner from the recording sheet side toward the intermediate transfer belt side, i.e., the positive peak value in the present example (hereinafter referred to as the returning peak value Vr). Vpp represents the peak-to-peak voltage.
Even if an AC bias including only an AC component is applied instead of the superimposed bias as illustrated in the drawing, it is possible to move the toner back and forth between the intermediate transfer belt 531 and the recording sheet P in the transfer nip. The AC bias, however, simply moves the toner back and forth, and is unable to transfer the toner onto the recording sheet P. If a superimposed bias including a DC component is applied to adjust the offset voltage Voff, i.e., the time-averaged value of the potential difference between the two rollers, to the same negative polarity as the polarity of the toner, it is possible to transfer the toner from the intermediate transfer belt side toward the recording sheet side during the back-and-forth movement thereof, and thereby to transfer the toner onto the recording sheet P.
The present inventors have observed the behavior of the toner in the transfer nip supplied with a superimposed bias including a DC component and an AC component as the transfer bias and found that, when the superimposed bias starts to be applied, only a very small number of toner particles present on a surface of a toner layer on the intermediate transfer belt 531 first separates from the toner layer and moves toward recesses of the recording sheet P. Most of the toner particles present in the toner layer remain therein. The very small number of toner particles having separated from the toner layer enters the recesses of the recording sheet P. Thereafter, if the direction of the electric field is reversed, the toner particles return from the recesses to the toner layer. In this process, the returning toner particles collide with other toner particles remaining in the toner layer, and reduce the adhesion of the other toner particles. Then, in the next reversal of the direction of the electric field to the direction for moving toner particles toward the recording sheet P, a larger number of toner particles than in the first cycle separates from the toner layer and moves toward the recesses of the recording sheet P. As the above-described sequence is repeated, the number of toner particles separating from the toner layer and entering the recesses of the recording sheet P is gradually increased. Consequently, a sufficient number of toner particles is eventually transferred to the recesses.
However, it was found that, if the absolute value of the returning peak value Vr illustrated in FIG. 2 is relatively small, it is difficult to cause the toner particles transferred into the recesses of the recording sheet P to return to the toner layer, and the toner particles remain in the recesses. This results in a failure to increase the number of subsequent toner particles and a deficiency in overall toner adhesion amount in the recesses. It was also found that the lower limit value of the returning peak value Vr required to transfer a sufficient amount of toner into the recesses varies depending on the depth of the recesses because the reverse electric field for causing the toner particles having entered the recesses to return to the toner layer needs to be increased in intensity in accordance with an increase in depth of the recesses. Therefore, the above-described lower limit value is increased. That is, the deeper the recesses of the recording sheet P, the larger the lower limit value of the returning peak value Vr required to transfer a sufficient amount of toner into the recesses. Therefore, to transfer a sufficient amount of toner into the recesses of the recording sheet P, if the recesses are very deep, the returning peak value Vr needs to be set to a very large value. As observed from the waveform of FIG. 2, however, the peak-to-peak voltage Vpp also needs to be increased to set the returning peak value Vr to a relatively large value. If the peak-to-peak voltage Vpp is increased, however, white spots attributed to uncontrolled electrical discharge tend to appear in the image. The white spots are attributed to discharge occurring across a gap between a bottom portion of the recesses of the recording sheet P and an image carrier, such as the intermediate transfer belt 531. Moreover, the higher the peak-to-peak voltage Vpp, the more easily discharge occurs. Further, the shallower the recesses, the more easily discharge occurs, provided that the peak-to-peak voltage Vpp is the same. Therefore, if the returning peak value Vr is set to a very large value to fully transfer the toner to a recording sheet with relatively deep recesses, the white spots attributed to discharge tend to appear in a recording sheet with relatively shallow recesses. At the same time, however, if the returning peak value Vr is reduced to minimize the appearance of white spots, it is difficult to transfer a sufficient amount of toner into the recesses of the recording sheet, if the recesses are relatively deep. As a result, an uneven density pattern is formed.